In order to stimulate and more effectively produce hydrocarbons from downhole formations, especially formations with low porosity and/or low permeability, induced fracturing (called “frac operations”, “hydraulic fracturing”, or simply “fracing”) of the hydrocarbon-bearing formations has been a commonly used technique. In a typical frac operation, fluids are pumped downhole under high pressure, causing the formations to fracture around the borehole, creating high permeability conduits that promote the flow of the hydrocarbons into the borehole. These frac operations can be conducted in horizontal and deviated, as well as vertical, boreholes, and in either intervals of uncased wells, or in cased wells through perforations.
In cased boreholes in vertical wells, for example, the high pressure fluids exit the borehole via perforations through the casing and surrounding cement, and cause the formations to fracture, usually in thin, generally vertical sheet-like fractures in the deeper formations in which oil and gas are commonly found. These induced fractures generally extend laterally a considerable distance out from the wellbore into the surrounding formations, and extend vertically until the fracture reaches a formation that is not easily fractured above and/or below the desired frac interval. Normally, if the fluid, sometimes called slurry, pumped downhole does not contain solids that remain lodged in the fracture when the fluid pressure is relaxed, then the fracture re-closes, and most of the permeability conduit gain is lost. These solids, called proppants, are generally composed of sand grains or ceramic particles that are placed in the induced fractures to keep them from fully re-closing. After the slurry is pumped downhole and the fluid pressure is released, the formation walls close on the propping agent creating a “propped fracture” which oftentimes provides a high conductivity channel in the subterranean formation. The time for fractures to close is formation dependent and is so far unable to be directly measured.
Although induced fracturing has been a highly effective tool in the production of hydrocarbon reservoirs, the amount of stimulation provided by this process depends to a large extent upon the ability to generate new fractures, or to create or extend existing fractures, as well as the ability to maintain open fractures through appropriate selection and placement of proppant. Reliable methods for detecting the closure time of fractures to confirming whether or not proppant selection and placement has been appropriate, are not available.
There is a need, therefore, for a method of detecting when and where a fracture closes to determine fracture closure time and the extent of fracture closure.